The invention relates to a polycarbonate copolymer of spirobisindan and bisphenol-A.
The invention also relates to a method of preparing such a copolymer.
The invention further relates to an optical component of such a copolymer, in particular a substrate for an optical information carrier, notably a substrate for a magneto-optical information carrier. The invention also relates to a magneto-optical recording medium.
Polycarbonate (PC) on the basis of 2,2-bis(4-hydroxyphenyl)propane, in literature commonly referred to as bisphenol-A, is used in optical products because it has excellent thermal and mechanical properties and a low water absorption. Moreover, the material is transparent for a broad wavelength range. Products such as substrates for optical information carriers are generally manufactured by injection moulding or compression moulding polycarbonate at an increased temperature (at least above the glass-transition temperature T.sub.g). Such moulding processes causes the polycarbonate to become anisotropic. This is because, during injection moulding or compression moulding of polycarbonate, large pressure and temperature gradients develop which lead to molecular orientation and stresses. During cooling in the mould, these stresses and molecular orientations are frozen-in, so that the polycarbonate becomes optically anisotropic. This manifests itself in birefringence of the material. In a birefringent material a light ray is split into two plane-polarized light rays (referred to as ordinary and extraordinary ray) whose planes of polarization extend perpendicularly to each other. Both light rays have a different refractive index in the material. The difference in refractive index .DELTA.n is referred to as birefringence. The difference in path length between the ordinary and extraordinary light ray is referred to as optical retardation T and is proportional to .DELTA.n. This birefringence leads to astigmatism of a light beam. Birefringence is particularly disadvantageous in MO (magneto-optical) recording, because reading of an MO medium is based on the detection of the very small (Kerr)rotation of the plane of polarization of a polarized laser light beam.
It has been found that in the polymeric melt (temperature&gt;T.sub.g) the birefringence (.DELTA.n) is proportional to the applied stress (.DELTA..sigma.): EQU .DELTA.n=C.sub.m .multidot..DELTA..sigma.
The stress-optical coefficient (C.sub.m) is proportional to the anisotropy of the polymer chains. The applied voltage causes the polymer chains to become oriented; the material becomes anisotropic, which leads to birefrigence. This orientation is frozen-in when the temperature falls below T.sub.g. Also in the glass state (temperature&lt;T.sub.g) there is a linear relation between the applied voltage and birefringence, however, the proportionality constant C.sub.g is different.
In an injection-moulded product the birefringence is predominantly caused by orientation of the polymer chains. This birefringence is referred to as orientation birefringence. Thermal stresses developing at a temperature below T.sub.g are generally small and relax with time. The contribution of the molecular orientation to the birefrigence depends on the degree of orientation of the polymer chains and the optical anisotropy inherent in polymer chains. The orientation can be reduced by employing high melting and mould temperatures and by using a polymer having a molecular mass which is as small as possible. The intrinsical anisotropy is determined by the molecular configuration and conformation, in particular the spatial distribution of the polarizable electrons. The polarizability of most polymer molecules is higher in the chain direction than in the direction perpendicular thereto. In this case, there is a positive anisotropy which manifests itself in a positive value of C.sub.m. The value of C.sub.m is a measure of the sensitivity of a polymer to orientation birefringence. It follows from the above that in order to obtain an optically intrinsically isotropic polymer the value of C.sub.m should be zero or substantially zero. Bisphenol-A-polycarbonate (Bis-A PC) has a positive, high C.sub.m value (+55.10.sup.-10 Pa.sup.-1 or +5500 Brewsters). Polymers having highly polarizable side groups, such as polystyrene, have a negative anisotropy.
An optically isotropic polymer can be obtained by mixing polymers having a positive anisotropy and polymers having a negative anisotropy in the proper ratio. A prerequisite condition is that the polymers must be miscible. For example, Bis-A PC and polystyrene are immiscible. Also copolymerization of the proper quantities of monomers having a positive anisotropy and monomers having a negative anisotropy can yield an optically isotropic material. Another way of reducing the intrinsical anisotropy of a polymer is by providing substituents on the polymer chains.
In European Patent Application EP-A-287887, a description is given of a polycarbonate on the basis of spirobisindan bisphenol, namely 6,6'-dihydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindan (SBI), hereinafter referred to as spiro-PC. In said European Patent Application, it is stated that a homopolymer of SBI has a birefringence equal to 0 and that copolymerization with bisphenol-A causes the birefringence to increase (Table III) and T.sub.g Table II) to decrease. However, said Patent Application does not state how the birefringence was determined. The polymer molecules may be oriented parallel to the substrate surface, so that, despite the fact that the material is anisotropic, a perpendicularly incident light beam exhibits a very small birefringence. However, under operating conditions, i.e. with a convergent light beam, birefringence does occur. A convergent light beam comprises, inter alia, rays having an angle of incidence of for example 30.degree., resulting in anisotropy manifesting itself.
Measurements carried out by Applicant have shown, however, that the homopolymer of SBI (spiro-PC) is not optically isotropic, but instead has a negative C.sub.m value of -650.10.sup.-12 Pa.sup.-1 (-650 Brewsters). In this homopolymer, a convergent light beam exhibits birefringence.